Ternary V₂O₅–WO₃/TiO₂ catalysts were prepared by sequential or simultaneous grafting steps of vanadia and tungsta onto titania and were compared with similarly prepared binary catalysts. Different grafting sequences including alternating grafting of vanadia and tungsta were compared.

During the grafting process the metal oxide precursor reacted with hydroxyl groups of the other grafted metal oxide. The interaction of vanadia with tungsta species resulted in a different reduction behavior of both metal oxides compared to the binary catalysts as indicated by temperature-programmed reduction with hydrogen. The strength of interaction of the grafted species depended on preparation sequence and metal loading.

At low coverage (‹monolayer) catalyst properties were found to depend strongly on loading, but relatively little on the grafting mode, as indicated by vibrational spectroscopy. Laser Raman experiments at different laser power revealed reversible effects due to temperature induced structural changes of surface vanadia species. For all catalysts, even at a loading of more than one and a half monolayers, no evidence of crystalline vanadia or tungsta could be found. After calcination of WO₃/TiO₂ catalyst at 1023 K instead of 573 K and subsequent grafting with vanadia, new species with hydroxyl groups showing a vibrational frequency below 3600 cm⁻¹ were formed. The increase of the calcination temperature had no significant influence on the reduction of vanadia by hydrogen.

The conformational behaviour of several α-ketoesters was investigated using solution FTIR in combination with ab initio calculations. The α-ketoesters show marked differences in the O=C–C=O torsional potential energy surface depending on the substituent at the α-keto group. In general the torsional potential is characterised by broad minima corresponding to s-cis and s-trans conformations and low interconversion barriers. The s-trans conformation is more stable but the fraction of s-cis is considerable at room temperature and increases with solvent polarity due to the higher dipole moment of the latter. Hydrogen bonding with alcoholic solvents also leads to a stabilisation of the s-cis conformer. The interaction of ethyl pyruvate with R₃N⁺–H is much stronger when ethyl pyruvate adopts an s-cis conformation due to strong ion–dipole interaction. This type of interaction between ethyl pyruvate and protonated cinchonidine is considered to be crucial for the enantio-differentiation in the heterogeneous enantioselective hydrogenation of ethyl pyruvate over cinchonidine modified platinum in acidic media.

IR and NMR experiments revealed that the enantioselective hydrogenation of ethyl pyruvate in nonacidic solvents is complicated by the simultaneously occurring self-condensation (aldol reaction) of the reactant. Both enantioselective reactions are catalyzed by the chiral base cinchona alkaloid, but the hydrogenation is faster by several orders of magnitude than the aldol reaction. Catalytic experiments proved that the aldol products are not spectator species. The enol form of the major aldol product protonates the quinuclidine N of cinchonidine and enhances the enantiomeric excess of the hydrogenation reaction. The significance of this observation with respect to kinetic and mechanistic studies is discussed.

The adsorption of ethyl pyruvate on Pt(111) has been studied by in situ XANES measurements in the presence and absence of hydrogen. Depending on the hydrogen and ethyl pyruvate pressure, the C and O K‐edge spectra exhibit distinctly different angular dependence. Without hydrogen ethyl pyruvate is oriented preferentially perpendicular to the surface, indicating bonding via the O lone pairs. In the presence of hydrogen the mean orientation is more tilted towards the surface. Likely, ethyl pyruvate also interacts with Pt via its π system under these conditions. The observed angle‐dependent shift of the energy of the π* and σ* resonances indicates the coexistence of differently adsorbed ethyl pyruvate species. The experimental findings demonstrate the importance of the in situ approach for unraveling the adsorption mode of ethyl pyruvate in the enantioselective hydrogenation over cinchona‐alkaloid‐modified Pt.

The adsorption of ethyl pyruvate on Pt(111) at low temperature was investigated by XP and UP spectroscopy. The assignment of the photoelectron spectra was assisted by calculation of correlated ionization potentials. Comparison of the XP and UP spectra of the condensed and chemisorbed layer indicates a strong ethyl pyruvate adsorption bond in the latter. Upon chemisorption, the HOMO of ethyl pyruvate, which is a lone-pair orbital delocalized over both C=O groups, is stabilized by about 0.7 eV with respect to the other orbitals, which is characteristic for a lone-pair bonding mechanism. The same bonding mechanism was found for coverages far below saturation. The XP spectra further indicate that the ketone C=O is more strongly involved in the chemisorption bond than the carboxyl C=O of ethyl pyruvate. The packing density of the saturated chemisorbed ethyl pyruvate layer, as determined by XPS, is high. This points toward an upright or tilted orientation of ethyl pyruvate in this layer, in line with the observed bonding mechanism.

Interaction complexes between cinchonidine modifier and methyl pyruvate reactant proposed for the enantioselective hydrogenation over platinum catalysts have been calculated using ab initio methods. For s-trans-methyl pyruvate it was found that the complex yielding (R)-methyl lactate upon hydrogenation was more stable than the corresponding pro-(S) complex. The calculated energy difference of 1.8 kcal/mol corresponds to an enantiomeric excess of 92%, in good agreement with experiment. For the analogous complexes of s-cis-methyl pyruvate the energy difference is only 0.2 kcal/mol in favour of pro-(R), corresponding to 17% enantiomeric excess. Due to the larger dipole moment of the s-cis conformer of methyl pyruvate its hydrogen-bonded complexes with cinchonidine are considerably more stable than the corresponding s-trans complexes. However, the predicted low enantiomeric excess for the s-cis conformer is in contrast with experiment. Possible reasons for this behaviour are discussed.

We present a theoretical analysis of the temperature dependence of the vanadyl pyrophosphate VO₂P₂O₇ ³¹P NMR spectra. Four distinct phosphorus sites responsible for four signals are identified in the crystal structure. The magnetic states of the crystal are described by two alternative models: the spin ladder and the dimer chain. Within both models, finite clusters with and without periodic conditions are considered. The fit of the experimental NMR data allows us to define combinations of hyperfine coupling parameters which are found to be similar in both spin models.

The total rate constant k₁ has been determined at P = 1 Torr nominal pressure (He) and at T = 298 K for the vinyl-methyl cross-radical reaction: (1) CH₃ + C₂H₃ ^→ Products. The measurements were performed in a discharge flow system coupled with collision-free sampling to a mass spectrometer operated at low electron energies. Vinyl and methyl radicals were generated by the reactions of F with C₂H₄ and CH₄, respectively. The kinetic studies were performed by monitoring the decay of C₂H₃ with methyl in excess, 6 < [CH₃]₀/ [C₂H₃]₀ < 21. The overall rate coefficient was determined to be k₁(298 K) = (1.02 ± 0.53) × 10^-10cm³ molecule^-1 s^-1 with the quoted uncertainty representing total errors. Numerical modeling was required to correct for secondary vinyl consumption by reactions such as C₂H₃ + H and C₂H₃ + C₂H₃. The present result for k₁ at T = 298 K is compared to two previous studies at high pressure (100-300 Torr He) and to a very recent study at low pressure (0.9-3.7 Torr He). Comparison is also made with the rate constant for the similar reaction CH₃ + C₂H₅ and with a value for k₁ estimated by the geometric mean rule employing values for k(CH₃ + CH₃) and k(C₂H₃ + C₂H₃). Qualitative product studies at T = 298 K and 200 K indicated formation of C₃H₆, C₂H₂, and C₃H₅ as products of the combination-stabilization, disproportionation, and combination-decomposition channels, respectively, of the CH₃ + C₂H₃ reaction. We also observed the secondary C₄H₈ product of the subsequent reaction of C₃H₅ with excess CH₃; this observation provides convincing evidence for the combination-decomposition channel yielding C₃H₅ + H. RRKM calculations with helium as the deactivator support the present and very recent experimental observations that allylic C-H bond rupture is an important path in the combination reaction. The pressure and temperature dependencies of the branching fractions are also predicted.

Present paper is an overview of our efforts during the past few years to understand complicated corelations of physical phenomena related to pressure in Fe(I1) solid state spin transition systems. Some principal results concerning p, T, λ-experiments are extracted. In the context of correlation of the crystallographic phase transition with simultaneous HS → LS relaxation and LS → HS photopopulation, we show the latest results: Brillouin and magnetic measurements on the crystal [Fe(pt₆](BF₆)₂.

Luminescence and energy transfer in [Zn_1-xRu_x(bpy)₃][NaAl_1-yCr_y(ox)₃] (x ≈ 0.01, y = 0.006 − 0.22; bpy = 2,2‘-bipyridine, ox = C₂O₄^2-) and [Zn_1-x-yRu_xOs_y(bpy)₃][NaAl(ox)₃] (x ≈ 0.01, y = 0.012) are presented and discussed. Surprisingly, the luminescence of the isolated luminophores [Ru(bpy)₃]²⁺ and [Os(bpy)₃]²⁺ in [Zn(bpy)₃][NaAl(ox)₃] is hardly quenched at room temperature. Steady-state luminescence spectra and decay curves show that energy transfer occurs between [Ru(bpy)₃]²⁺ and [Cr(ox)₃]^3- and between [Ru(bpy)₃]²⁺ and [Os(bpy)₃]²⁺ in [Zn_1-xRu_x(bpy)₃][NaAl_1-yCr_y(ox)₃] and [Zn_1-x-yRu_xOs_y(bpy)₃] [NaAl(ox)₃], respectively. For a quantitative investigation of the energy transfer, a shell type model is developed, using a Monte Carlo procedure and the structural parameters of the systems. A good description of the experimental data is obtained assuming electric dipole−electric dipole interaction between donors and acceptors, with a critical distance R_c for [Ru(bpy)₃]²⁺ to [Cr(ox)₃]^3- energy transfer of 15 Å and for [Ru(bpy)₃]²⁺ to [Os(bpy)₃]²⁺ energy transfer of 33 Å. These values are in good agreement with those derived using the Förster−Dexter theory.

The crystal chemistry of the Sm³⁺ to Sm²⁺ reduction in tetraborate lattices was investigated. In crystalline SrB₄O₇ in air it is mainly Sm²⁺ that is incorporated from a melt or glass containing predominantly Sm³⁺. For the process in air, a reduction and pick-up mechanism is assumed to take place at the crystal/nutrient interface. Stabilization of Sm²⁺ in SrB₄O₇ at high temperature and in an oxidizing atmosphere seems to be a particular property of the system, because no Sm²⁺ inclusion could be observed along the series MB₄O₇ (M = Ca, Ba, Cd, Pb), if similar reaction conditions were applied. So far, there is only one other oxide lattice (BaB₈O₁₃) known where at high temperatures significant amounts of Sm²⁺ are obtained for reactions in the air.Single crystals of SrB₄O₇ : Sm²⁺ were grown by the Czochralski method (k_eff for Sm is 0.5). Optical hole burning experiments for the transition ⁵D₁–⁷F₀ were performed at 80 K. A hole with a width of 0.21 cm^–1 and a depth of 5.25% was formed for the first time for Sm²⁺ in a borate crystal excited by the beam of a single frequency dye laser. A rather small inhomogeneous linewidth of 0.28 cm^–1 allowed the burning of a single hole only.

We have investigated the luminescence of CaF₂ thin films doped with very low concentrations of Sm²⁺ ions using scanning confocal optical microscopy at low temperatures. The film morphology was studied independently by atomic force microscopy. The Sm²⁺ ions are homogeneously distributed in the films and show photobleaching. Unexpectedly, on the film surface strongly luminescent small topographic features are observed that are found to contain Sm³⁺ by spectral analysis. The formation of Sm³⁺ is probably due to the presence of oxygen during film growth. In the lowest doped films on-off blinking behavior of isolated luminescent spots provides strong evidence for the first observation of single ions in a crystal.

Crystals of ordered Ba₆EuF₁₂Cl₂ were found to form during high temperature flux growth. The structure was refined in the hexagonal space group P 6 to R^F(R^F_W) = 0.024(0.024) for 326 reflections and 46 parameters. Lattice parameters are a = b = 1059.27(8) pm and c = 416.36(2) pm; Z = 1. The structure is isotypic to Ba₇F₁₂Cl₂. No solid solution of Ba/Eu was observed, the Eu²⁺ ions are located in the channels formed by 3 + 6 fluorine ions, occupying only one of the three metal sites of the Ba₇F₁₂Cl₂ structure.

We recently discovered a new compound with composition Ba₇F₁₂Cl₂. It was possible to show that the variation of the synthesis conditions makes it possible to obtain a disordered and an ordered modification with different lattice parameters and space groups (P6₃/m [176] and P6  [174]). For Pb₇F₁₂Cl₂ an ordered modification is reported in the literature. In this paper we present the synthesis and structural characterization from X-ray diffraction data of the disordered modification of Pb₇F₁₂Cl₂. Single crystals were grown from a flux and the structure was refined in the hexagonal space group P6₃/m to R(R_w)=0.043(0.038) for 284 reflections and 26 parameters. Lattice parameters are a=b=1021.90(8) pm and c=361.93(6) pm with Z=1. Propeller-type arrangements with chlorine as axis and fluorines as blades are observed. The ordered modification of Pb₇F₁₂Cl₂ was prepared by a new hydrothermal synthesis. Differences between both modifications are found in the lattice constants and atomic occupation parameters for the atom type Pb2 and the connected fluorine ions.

Two distinct methods were used to investigate the role of Trp residues during Mg-ADP binding to cytosolic creatine kinase (CK) from rabbit muscle:  (1) Raman spectroscopy, which is very sensitive to the environment of aromatic side-chain residues, and (2) reaction-induced infrared difference spectroscopy (RIDS) and photolabile substrate (ADP[Et(PhNO₂)]), combined with site-directed mutagenesis on the four Trp residues of CK. Our Raman results indicated that the environment of Trp and of Tyr were not affected during Mg-ADP binding to CK. Analysis of RIDS of wild-type CK, inactive W227Y, and active W210,217,272Y mutants suggested that Trp227 was not involved in the stacking interactions. Results are consistent with Trp227 being essential to prevent water molecules from entering in the active site [as suggested by Gross, M., Furter-Graves, E. M., Wallimann, T., Eppenberger, H. M., and Furter, R. (1994) Protein Sci. 3, 1058−1068] and that another Trp could in addition help to steer the nucleotide in the binding site, although it is not essential for the activity of CK. Raman and infrared spectra indicated that Mg-ADP binding does not involve large secondary structure changes. Only 3−4 residues absorbing in the amide I region are directly implicated in the Mg-ADP binding (corresponding to secondary structure changes less than 1%), suggesting that movement of protein domains due to Mg-nucleotide binding do not promote large secondary structure changes.

In this comment, it is pointed out that the generalized gradient approximation (GGA) functionals considered by Milet et al. [ J. Chem. Phys. 111, 7727 (1999)] share the same exchange part (B88) which violates significantly the Lieb–Oxford bound. Violation of this exact condition was shown to result in significant errors of the exchange energy in the case of weakly overlapping electron densities [Wesołowski et al., J. Phys. Chem. A 101, 7818 (1997); Zhang et al., J. Chem. Phys. 107, 7921 (1997)]. Numerical examples are given to illustrate that such exchange functionals which better satisfy the Lieb–Oxford bound lead to better interaction energies also for the complexes studied by Milet et al.

The use of hybrid ab initio QM/MM methods in studies of metalloenzymes and related systems presents a major challenge to computational chemists. Methods that include the metal ion in the quantum mechanical region should also include the ligands of the metal in this region. Such a treatment, however, should be very demanding if one is interested in performing the configurational averaging needed for proper calculations of activation free energies. In the present work we examine the ability of the frozen DFT (FDFT) and the constrained DFT (CDFT) approaches to be used in ab initio studies of metal-catalyzed reactions, while allowing for an effective QM (rather than a QM/MM) treatment of the reacting complex. These approaches allow one to treat the entire enzyme by ab initio DFT methods, while confining the SCF calculations to a relatively small subsystem and keeping the electron density of the rest of the system frozen (or constrained). It is found that the FDFT and CDFT models can reproduce the trend obtained by a full DFT calculation of a proton transfer between two water molecules in a (Im)₃Zn²⁺(H₂O)₂ system. This and related test cases indicate that our approximated models should be capable of providing a reliable representation of the energetics of metalloenzymes. The reasons for the special efficiency of the FDFT approach are clarified, and the strategies that can be used in FDFT studies of metalloenzymes are outlined.

The mechanism of the protonation of ferrocene, the simplest model for the electrophilic attack on a metallocene, has been studied extensively. However, neither experiment nor computation have reached agreement on the details of the mechanism. The different model calculations applied [Hartree–Fock, Möller–Plesset, and density functional theory (HF, MP2, and DFT) with different functionals] come to contradicting conclusions. As a complement to our previous work, we report the results obtained for neutral and protonated ferrocene using the coupled-cluster method [CCSD(T)] with polarized double- and triple-zeta basis sets. These calculations show that the metal-protonated and the agostic forms represent minima on the potential energy surface, whereas the ring-protonated form is higher in energy with no minimum structure identified. With regard to the reaction, these results indicate an exo reaction path. The CCSD(T) results are in good agreement with the predictions made by the DFT calculations, whereas the results obtained from the Hartree–Fock and MP2 computations appear to be incorrect.

Incorporation of [Co(bpy)₃]²⁺ into the cavities of the three-dimensional oxalate network structure in [Co(bpy)₃][LiCr(ox)₃] produces chemical pressure that destabilises the normal high-spin ground state ⁴T₁ to such an extent that the [Co(bpy)₃]²⁺ complex becomes a spin-crossover complex. It shows a temperature-dependent equilibrium between the ²E low-spin and the ⁴T₁ high-spin states.

The dynamics of charge recombination within geminate ion pairs formed by electron transfer (ET) quenching of excited aromatic hydrocarbons by aliphatic and aromatic amines was investigated using picosecond transient grating spectroscopy. With aliphatic donors, the rate constant of back ET, k_BET, shows a substantial decrease with increasing steric encumbrance around the N atom. No correlation between k_BET and the exergonicity of the process was observed. This effect is ascribed to a decrease of the electronic coupling matrix element, V, which is affected by both the distance between the N atom of the donor and the aromatic plane of the acceptor and by the delocalization of the hole upon increasing the bulkiness of the alkyl substituents. With aromatic amines, k_BETis substantially slower than with the unhindered amines. This is also explained in terms of a smaller value of V because of charge delocalization.

The deuterium isotope effect on the fluorescence lifetime of contact ions pairs (CIP) composed of 9,10-dicyanoanthracene and weak aromatic electron donors in acetonitrile has been investigated. For all pairs studied, an increase of the emission lifetime with increasing deuterium substitution on both the electron acceptor and the electron donor was observed. The free ion yield shows the same variation upon deuteration. It is concluded that the efficiency of free ions formation is essentially determined by the competition, within the CIP, between charge recombination and dissociation into free ions and that loose ion pairs do not play a significant role.

The radical anion (tmbp)^•-, where tmbp = 4,4‘,5,5‘-tetramethyl-2,2‘-biphosphinine, was generated by reduction of tmbp on a potassium mirror. EPR/ENDOR spectra and DFT calculations show that, in contrast to the neutral species, this anion is planar and that the unpaired electron is mainly delocalized on the PCCP fragment with a large participation of the phosphorus p_π orbitals. This planar structure was confirmed by the first crystal structure of an anionic biphosphinine:  [tmbp][Li(2.2.1)]. Reduction of [Ni(tmbp)₂] led to the 19-electron complex whose g and ³¹P hyperfine tensors were obtained from EPR in liquid and frozen solutions. These results, together with DFT calculations on [Ni(bp)₂] and [Ni(bp)₂]^•-, indicate that, by accepting an extra electron, the neutral nickel complex distorts toward a more planar geometry and that the dihedral angle between the two phosphinine rings of each ligand slightly increases. In the reduced Ni complex, the unpaired electron is mainly delocalized on the ligands, in a molecular orbital which retains the characteristics of the SOMO found for the reduced isolated ligand. A charge decomposition analysis (CDA) shows that, in [Ni(bp)₂], metal−ligand back-donation strongly contributes to the metal−ligand bonding.

The g, ³¹P and ¹H hyperfine tensors of the dibenzobarrelene phosphinyl radical, trapped in an X-irradiated single crystal of dibenzobarrelene phosphine, were estimated at 45 and 300 K. They indicate that among the three locations of the phosphinyl hydrogen expected from DFT calculations, only two are occupied at 40 K and that the third one remains practically vacant, even at 300 K. The temperature dependence of the EPR spectrum was simulated by assuming jumps between two P–H bond orientations (energy barrier ~= 0.5 kcal mol⁻¹) which correspond to the conformation of the PH₂ moiety in the only rotamer present in the dibenzobarrelene phosphine crystal.

Electron paramagnetic resonance, electron-nuclear double resonance, and optical spectroscopy of the tetragonal Yb³⁺ center in KMgF₃ are reported here. The results of these experiments allow us to conclude that a previously given structural model as well as the interpretation of the optical spectrum of this center are incorrect. A model is presented and experimentally and theoretically justified. In particular, the values of the hyperfine and transferred hyperfine interaction parameters were determined as well as an experiment-based energy-level scheme. Its parametrization is performed by including simultaneously the crystal field and the spin-orbit interaction within the ⁷F term. Furthermore, a theoretical analysis of the transferred hyperfine interaction (THFl) parameters is presented. It is further shown from optics and from microscopic calculations of the THFI parameters that g_∥ and g_⊥have opposite signs and that the rule of correspondence between the cubic g factor and g̃=1/3(g_x+g_y+g_z) does not depend on the relative magnitude of the cubic and low-symmetry crystal field acting on the rare-earth ion.

The optical and paramagnetic properties of X-irradiated silver doped SrF₂ crystals were investigated. The freshly irradiated crystals show a complex absorption spectrum between approximately 200 and 650 nm. Subsequently, systematic heat treatments were applied and absorption, photoluminescence and its polarization dependence, thermo- and radio-luminescence experiments have been undertaken. The resulting experimental data were mutually correlated with the aid of the factor analysis technique and six different origins of the observed spectra were identified. Models of the underlying silver-defect structures are discussed and crystal preparation is further presented.